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Eye disorders and Zebrafish
The zebrafish model system
Among a range of eye diseases, Retinitis Pigmentosa (RP) is a serious disease that affects the central function of the eye, the light response. RP is a misnomer, as the word retinitis implies an inflammatory response. However this is not the predominant feature of this condition. Today the condition would probably have been named "Retinal Dystrophies" but the old name is well known throughout the world, so it stays.
RP is a rare (1:5000) group of eye diseases in which there is damage to the retina of the eye. The retina is a light sensitive tissue, which lines the inside of the eye at the back where the first stages of seeing take place. The eye is a glass-like sphere, with a lens at the front which focuses light onto the retina.
The retina consists of two main layers. Light first travels through a thin pigment epithelium, which performs a number of functions required for the retina to work properly, such as blood supply. Then the thicker neural retina holds the photoreceptor (light responding) cells, which are arranged in layers. Although it may seem strange that this layer is furthest from the incoming light, it is an arrangement found in all animals with backbones.
There are two types of photoreceptor cells: rods and cones. The cone cells are responsible for seeing colour and are concentrated at and around the centre of focus of the ey's lens. The rod cells are more sensitive and therefore used in poor light conditions for monochrome sight. These cells are located in greater concentrations towards the edges of the retina. There are about 120 million rod cells but only 6 million cone cells in the human eye.
Light triggers the rod and cone cells to send an impuls to a layer of intermediate neurones. These nerve cells connect to a next layer of ganglion cells that have long fibres which join to form the optic nerve and carry the signal to the brain. This is where the picture is assembled, the process we call seeing.
The first signs of retinitis pigmentosa usually occur in early childhood, when both eyes typically are affected. Many patients with RP also experience photopsiae as the disorder progresses; typically they report small flashes of light or a twinkling, shimmering sensation in the midperipheral or peripheral field. These are believed to represent aberrant electrical impulses from the degenerating retina. The main sign of the disease is the presence of dark pigmented spots in the retina.
The retinal defect may be found in the retina's rod cells, the cone cells and/or in the connection between the cells that compose the retina. The cells controlling night vision (rods) are most likely to be affected. A shrinking field of vision is one of the early warning signs of retinitis pigmentosa. The disorder will continue to progress, although slowly. During later stages of retinitis pigmentosa, only a small area of central vision remains, along with slight peripheral vision. Complete blindness is uncommon.
With advances in molecular research, it is now known that RP constitutes many retinal and pigment epithelium dystrophies, which may be caused by molecular defects in more than 100 different genes. Therefore RP is a complex inherited disease. A complicating factor is the fact that patients with the same mutation can phenotypically have different disease manifestations.
Unfortunately, to date, there is no specific treatment for retinitis pigmentosa. However, protecting the eye's retina with the use of UV sunglasses may be helpful in delaying the onset of symptoms.
